In RStudio IDE there are many file types you can use from a "New File" menu. Unfortunately it happens that quite commonly I have to create just a "Text file" and change extension afterwards.
The types I found missing on that lists, which I quite commonly use are:
- markdown - ".md"
- C - ".c"
- header - ".h"
- shell - ".sh"
The last one, "sh", may not be that much relevant to R but "md" and "c"+"h" are quite heavily relevant for R developers, thus IMO should be added to this list.